Gas energized, gravity flow wall furnaces have been widely used for many years to provide heat for one or two rooms, typically in structures not having central heating. These furnaces are usually partially recessed into a wall in the space between two studdings in a conventional stud wall. Such space is normally only about 143/8" wide. The furnace must also be very shallow since a conventional stud only provides a space about 31/2" in depth and the furnace usually extends only another 3"-4" into the room. As a result of these dimensional constraints and because of cost, many such wall furnaces do not have a fan and simply rely on gravity for the flow of room air and combustion products. That is, the cool room air sinks and the warm room air and the hot combustion gases rise.
Current wall furnaces typically include a thin, flat, wide heat exchanger extending vertically in the wall, with the edges of the exchanger being positioned adjacent to but spaced from the studs in the wall. The combustion gases from a burner positioned at the lower end of the heat exchanger are ducted upwardly through the heat exchanger to a flue. Room air flows through a grill forming the wall of the furnace facing the room to be heated. Cool room air enters the furnace near the lower end of the heat exchanger, is heated from the exterior of the heat exchanger, and flows upwardly due to a decrease in density caused by heating, and exits back into the room at the upper end of the heat exchanger. With this simple arrangement, fairly effective heat transfer is obtained.
Typically, the highest thermal efficiency provided by such gravity flow wall furnaces has been about 70%. This roughly means the combustion process itself is fairly complete and that about 70% of the heat from the combustion gases is transferred into the room. This puts further constraints on the construction of the heat exchanger in that the combustion gases must flow upwardly through the heat exchanger with sufficient velocity to ensure that adequate air is drawn into the burner to provide sufficient oxygen and to produce a continued flow.
At the same time, it is desirable that the velocity of the combustion gases be sufficiently slow to maximize the heat transfer from the heat exchanger. The heat exchangers currently being used are quite thin and flat at their upper ends and usually include a plurality of interconnections between the front and back walls to impede flow, and thereby improve heat transfer to the room air flowing over the exterior surfaces of the exchanger.
Further constraints on the design and size of the furnace are that the temperatures of the walls surrounding the furnace and of the grill exposed to the room being heated must be kept to certain minimums to satisfy fire and safety requirements. Also, efficiency is reduced by th fact that some room air is allowed to enter a vent hood at the top of the heat exchanger to ensure the temperature of the combustion gases as they leave the furnace do not exceed a certain maximum.
In recent years, a further requirement involving conservation of energy has been governmentally mandated. This was primarily imposed with respect to forced air, central, gas heating systems, but the regulations are currently being interpreted to be applicable as well to wall furnaces. The 70% thermal efficiency rating of current wall furnaces does not satisfy this requirement. Thus, a need exists for an improved heat exchanger for a wall furnace that will improve efficiency and meet the various standards established. These standards must be met based on a gravity flow system, i.e. without the use of a fan to circulate room air or a fan to induce draft for the combustion process. Of course, such improvement must also be practical and inexpensive in order to be competitive from a marketing standpoint.